Lady listening to music

Headphones in the digital age

Developments in headphone technology are being driven by growing consumer demand for ever-higher fidelity and style.

The era of mass-market audiophilia has led to some big changes in headphone technology, a long-established audio staple which some might have thought had reached the limits of innovation. Yet new developments keep coming, and sales stay relatively healthy while the electronics market generally has been hit by economic insecurity. Market-leading European audio products vendor Sennheiser provides an indication of market growth from its revenues: last June it posted a 20 per cent increase in turnover, of which its headphone business accounted for the largest part – 34.9 per cent, up from 33.8 per cent the previous year. Small growth, but growth all the same.

In August 2011 smartphone manufacturer HTC paid £190m for a 51 per cent stake in Beats Electronics, the company that manufactures the high-end Beats headphones by rapper Dr Dre. The Beats headphones claim to provide a higher performance audio and the company's aim is to make them a serious market competitor.

Innovation in headphone technology dates from the 1930s, when German audio company Beyerdynamic claimed to have invented the first 'dynamic headphones'. In the 1950s the Beyerdynamic Dynamic Telephone 48s were the world's first high-fidelity headphones, and they are still popular among video and sound production professionals today.

According to the 'World of Invention', jazz musician John C Koss invented stereo headphones in 1958. With assistance from an audio engineer, Koss put together a complete audio listening system with a portable phonograph, speakers and headphones, creating what was arguably a prototype of the portable personal audio player that led to the iPod. The Koss headphones found a market among musicians and music aficionados, as well as becoming part of the communications set-up used by the military. This success led to other companies – most notably in Japan – adopting the design and developing innovative products of their own.

The global habit of listening to music and other audio content via mobile devices, laptops and tablets has created an annual market worth £415m, and growth is further driven by consumers' willingness to spend more money on accessories.

According to market analyst NPD, headphones costing $100 (£61) or more grew from around 2 per cent of the market in 2009 to 3.5 per cent in 2010, with the average user replacing them every 14 months, either because of fashion, functionality, damage or loss. According to the Consumer Electronics Association, in 2010 the total number unit sales to dealers of headphones and earphones was 74,638. The estimated total for 2011 is 79,369, and the forecast for 2012 is 83,561.

'It is quite easy for people to say 'it's just a bit of wire, a jack, and small speakers, how different can they be?', but the quality does vary a lot,' says Sennheiser product manager Dom Feeney. 'The sound and build quality varies due to the type of components used and the inherent design principles. Sennheiser use transducers which lead to better sounds than the standard headphone.'

Core technology

The mechanics of the speakers start with a cone, which has a diaphragm of paper, fabric or plastic stretched across the wide part and fastened to a metal ring. The smaller end contains an iron coil sitting in front of a magnet which is connected to the wires that connect the speaker to the player device.

The sound from the device is translated into electrical signals that convert the coil into an electromagnet. The magnetised coil either attracts or repels the magnet behind it, which moves the coil back and forth. The motion of the coil pushes and pulls the diaphragm, creating waves and pumping sound waves out from the speaker straight into the ear. The human ear depends on the eardrum, which vibrates in response to sound waves. The brain then translates the vibrations into what the listener experiences as sound.

Open-back headphones are designed to allow sound out through the back of the ear cup. The headphones sit lightly on or over the ear, and are typically lightweight. The design gives character to the sound but results in bass leakage and a spill-in of noise. Many users prefer this style as it offers a more natural sound.

Closed-back headphones are designed to reduce the effect of external noise and prevent the noise from leaking out. Typically, DJs and studio monitors would use these types of headphones, often called 'circumaural'. The design of the ear cups seals the ears from external noise with semi-airtight ear cups. These headphones are beneficial for those wanting to keep information discreet, and the ear cups reflect and boom the sound made from headphone drivers. Closed back styles have seen a rise in popularity among users who travel through noisy metropolitan environments.

Noise-cancelling designs

The concept of noise-cancelling headphones is not simply a case of the headphones having good physical sound barriers. A technology called active noise control essentially erases or destroys unwanted sounds before they reach the ear. Using noise-cancelling headphones on a flight, for example, will screen out the drone of the aeroplane engine.

Traditional headphones typically contain a pair of small speakers; noise-cancelling headphones, however, require more intricate technology. To eliminate incoming noise, the headphones need to identify and evaluate the soundwaves approaching the listener. The process involves having a microphone inside or outside the ear cups – usually microphones inside the ear cups perform better because they sense the waves that the listener is hearing.

The sounds gathered are sent to a circuit board in the headphones which analyses it and instantaneously formulates an opposing wave. The cancelling soundwave is usually released from a secondary driver, a separate speaker that intercepts and cancels out incoming noise. The changing flow of incoming sounds is monitored and the appropriate cancelling waves are generated.

'A question I'm often asked is whether we are hiding or actually decreasing sound,' says Dan Gauger, research manager in noise reduction technology at audio firm Bose. 'The answer is that we are decreasing it. The microphone constantly senses changes in the air density – sound – and the electronics determine whether you want to hear those changes. We destroy the sound you don't want to hear.'

Wireless connection

Traditional headphones were tethered to the device with a cable, which fed them the signal. Wireless headphones give the user the freedom of movement by eliminating cables and wires.

Techniques for transmitting a signal from device to headphones have varied since they were introduced. There are two main groups of wireless headphones: radio frequency (RF) and Bluetooth. RF headphones use a radio signal and are more powerful, using FM frequency rather than digital, so they work like a radio. Bluetooth headphones use a lower-power radio signal and digitally transmit the sounds.

Another type, infrared headphones, use an infrared beam to transmit sound from the base unit to the headphone, working in the same way as a remote control and a TV. Range is limited to 7m, and you must have a clear line-of-sight between transmitter and headphone.

RF has been the preferred method of transporting communications wirelessly for a long time. The RF signals work as invisible electronic waves, which transmit over air to different locations at fast speeds.

The wavelength of the electromagnetic signal is proportional to the frequency, so the higher the frequency the shorter the wavelength. The RF wireless headphones usually operate in the 800-900MHz band but will eventually move into higher frequencies such as 2.4GHz and 5.8GHz. These signals require two components – a transmitter to transform sound into an electromagnetic signal to either a single or multiple hosts, and a receiver to read the electromagnetic signal and translate it into sound. The transmitter enables communication to be sent between varied distances. The range required for headphones is usually keeping within 100-300m of the transmitter.

Bluetooth starts to bite

Bluetooth was designed by telecoms vendor Ericsson in 1994 to create simple and cost-efficient wireless communication pieces that would work with mobile phones, using short wavelength radio transmissions in the ISM band from 2400-2480MHz.

Headphone manufacturers such as Samsung and Sony have seen the potential in creating smaller and agile headphones that connect using Bluetooth. In order for the devices to connect, Advanced Audio Distribution Profile Bluetooth is required to allow a stereo-to-headphone connection. In August 2011, Sony launched what it claimed are the first wireless headphones to offer digital surround sound capabilities. The MDR-DS7500 features Sony's audio processing chip for surround-sound audio, with support for up to 7.1 channel audio. It also has Dolby ProLogic IIz for surround sound reproduction from stereo signals.

Wireless technology company Cambridge Silicon Radio (CSR) specialise in the user experience in varied electronic products. CSR's Bluetooth, audio and Wi-Fi technologies are used in well-known brands, such as HTC and Nokia. 'Typically, music devices like the iPod have a playback time of 12-14 hours, so headphones which run on Bluetooth need low-power technology to support the time length,' says senior product marketing manager at CSR Paul Wilson, 'but wired headphones are still common as they are cheaper to manufacture.'

Wilson explains: 'We provide headphones with both wired and wireless functionalities because consumers use their smartphones for TV, music and gaming, not just communication. The big market change is audio quality and user experience. We want our consumers to have the ability to switch to wired or wireless easily using one headphone, and now tablets like iPad and Samsung Galaxy are popular so more is spent on high-quality headphones.'

Wilson adds that the demographic for headphone wear is teenagers who want to be noticed, especially if they are wearing brands like Dr Dre Beats and Skull Candy – teenagers see this as a cool fashion.

Manufacturer Becker Avionics offers Polycon wireless for law enforcement and special mission operations. The Polycon wireless intercom extension system enables the rotary and fixed-wing operators to have audio communications between crew members in and away from aircraft. The service eliminates communication restrictions by providing hands-free, wireless communications to all users. Pilots and off-aircraft flight paramedics preparing patient transportation and evacuation can remain in constant communication.

During the 1970s, electronic headphones were replacing telephone tubes for passenger entertainment on flights. Though there was an improvement in headphones, they were masked by cabin noise and distortion, which caused passengers to increase the volume settings. In 1978, Dr Amar Bose – founder of the Bose consumer and professional audio products brand – experienced these issues firsthand while on an international flight.

Bose believed the solution to achieve noise reduction in headphones was a closed-loop feedback control. He initiated a research program to study an array of acoustic, electronic and mechanical challenges in connection with active noise-cancellation headsets. This study led to the development of Bose consumer headphones and military headsets. The company was the first to introduce active noise-cancelling headsets to the aviation sector over 20 years ago. The Bose A20 aviation headset has been re-engineered significantly for increased noise reduction in loud environments. *

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